1. J. Behrens, B.-A. Jerchow, M. Würtele, J. Grimm, C. Asbrand, R. Wirtz, W. Birchmeier, Max Delbrück Center for Molecular Medicine, Robert-Rössle-Strasse 10, 13122 Berlin, Germany.
2. M. Kühl and D. Wedlich, University of Ulm, Albert-Einstein-Allee 11, 89081 Ulm, Germany.